New research suggests that higher ocean temperatures and coral bleaching are also triggering rapid behavioral shifts in reef fish.

Over the course of two years, an international team of researchers spent more than 600 hours underwater observing butterflyfish, a species that is considered a key indicator of coral reef health, both before and after a global coral bleaching event in 2016.

The researchers found that aggressive behavior had decreased in butterflyfish by an average of two thirds, with the biggest behavorial changes observed on reefs where bleaching had killed off the most coral.

New research suggests that coral bleaching triggers rapid behavioral shifts in reef fish.

Higher ocean temperatures caused by global warming are the leading cause of the mass bleaching events that have hit many of Earth’s coral reef systems in recent years. Research has shown that some reefs are losing the ability to bounce back from repeat disturbances, and now there’s evidence that the reefs themselves are not the only part of reef ecosystems that are subject to more long-term effects due to coral bleaching.

Over the course of two years, an international team of researchers led by Dr. Sally Keith of Lancaster University in the UK spent more than 600 hours underwater observing butterflyfish, a species that is considered a key indicator of coral reef health. In a study published in the journal Nature Climate Change last week detailing their findings, Keith and team write that while mass coral bleaching events are known to cause population declines in reef fish, their impacts on behavior are more poorly understood.

The research team applied principles of behavioral theory and community ecology to their observations of 5,259 encounters between individuals of 38 different Chaetodon (butterflyfish) species on 17 reefs across the central Indo-Pacific region, including in Indonesia, Japan, the Philippines, and Christmas Island. These observations were recorded both before and after the global coral bleaching event that hit reefs in the region in 2016, and the researchers say they show that bleaching-induced mass mortality of corals can cause significant changes in the behavior of coral-feeding fishes.

“We observed that aggressive behaviour had decreased in butterflyfish by an average of two thirds, with the biggest drops observed on reefs where bleaching had killed off the most coral,” Dr. Keith said in a statement. “We think this is because the most nutritious coral was also the most susceptible to bleaching, so the fish moved from a well-rounded diet to the equivalent of eating only lettuce leaves — it was only enough to survive rather than to thrive.”

The researchers found that encounters between pairs of butterflyfish were most likely to be aggressive when both fish were coral-feeding specialists on reefs with high coral cover. They also discovered that, after a bleaching event had occurred, the amount of Acropora coral in the butterflyfish diet decreased significantly, with the fish taking up to 85 percent fewer bites of the nutritionally rich coral and not compensating for that loss with higher overall bite rates.

“The observed reduced aggression at low resource levels due to nutritional deficit follows the predictions of the economic theory of aggressive behaviour,” Keith and her co-authors write in the study. “Our results reveal synchronous changes in behaviour in response to coral mortality. Such changes could potentially disrupt territories, leading to reorganization of ecological communities.”

These behavioral changes could be driving other, more obvious, changes, the researchers added, including declining numbers of fish individuals and species on coral reefs. And the team suggested that their findings might help explain the mechanisms driving population declines in similarly disrupted ecosystems worldwide.

Study co-author Nathan Sanders, an ecologist at the University of Vermont in the United States, said that the team’s findings could also have implications for how we monitor the health of coral reef ecosystems: “This matters because butterflyfishes are often seen as the ‘canaries of the reef’ due to their strong reliance on coral, they are often the first to suffer after a disturbance event.”

Monitoring butterflyfish behavior might therefore provide scientists with an early warning system, helping predict bigger impacts to come, according to study co-author Dr. Erika Woolsey of Stanford University in the US.

“Our work highlights that animals can adjust to catastrophic events in the short term through flexible behaviour, but these changes may not be sustainable in the longer-term,” said co-author Andrew Baird of the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia.